The field of the invention generally relates to semiconductor fabrication techniques and the apparatus for implementing same. In greater particularity, it refers to an apparatus and method for reducing the deleterious effects of ion implantation in a semiconductor sample. In still more greater particularity, the apparatus and method of this invention concern an improved annealing of a semiconductor surface that reduces the deleterious effects of ion implantation in a semiconductor by assuring a precisely maintained annealing temperature for a precisely measured period of time for a sample being annealed after an ion implantation procedure.
Ion implantation is an established method for changing the conductivity (doping) of a semiconductor. It has been used to create a uniformly conductive layer with a thickness on the order of a few tenths of a micron or to create islands of conductivity within a seal of semi-insulating material. This latter capability has demonstrated great utility for ion implantation in the fabrication of high speed planar integrated circuits in semi-insulating materials such as gallium arsenide (GaAs) or indium phosphide (InP).
A pure ion beam of the proper dopant is directed onto a sample surface during a selective implantation procedure. The sample surface previously has been masked to allow the beam to strike some areas and not others. The ions penetrate beneath the semiconductor surface where they remain. The semiconductor, initially a single crystal, usually is heavily damaged during the implant procedure. In fact, at heavy ion doses, it is well established that the semiconductor will become amorphous in the implanted region. Following the implant, the mask is removed and the sample is annealing by raising it to a temperature in the range of 700.degree. to 1000.degree. C. The annealing accomplishes two things: first, it removes most of the damage; and, second, it makes the implanted impurities electrically active. However, even though a conventional annealing process does provide these advantages, the anneal is the primary source of difficulties in the ion implantation process. This also holds true even for silicon which can easily withstand such temperatures.
The reason for the difficulty is that the implanted impurities tend to migrate in an uncontrolled fashion during long lasting anneals in a furnace. Particularly in the case of gallium arsenide and indium phosphide compound semiconductors which can be made more reliably semi-insulating than silicon, there are in addition the more severe problems associated with substrate decomposition that is due to the loss of arsenic or phosphorus at high temperatures. Consequently, the gallium arsenide and indium phosphide samples must be protected during the annealing procedure. One way the samples are protected is to provide a dielectric cap that is deposited on the sample surface to help prevent decomposition.
Reduction of the annealing time has, in general, proven to be beneficial in minimizing the problems associated with the anneal. An indium phosphide wafer with a thickness of 0.020 inches has a thermal time constant of about 10 milliseconds, and other semiconductors in water form will respond about this rapidly to a temperature change. Consequently, a variety of methods have evolved for rapid annealing of semiconductor samples. A typical effort is the "Furnace Transient Anneal Process" of David A Collins et al in their U.S. Pat. No. 4,555,273. This process uses a movable furnace which is rapidly rolled over a sample resting on a low thermal mass support. It is a relatively uncomplicated process which operates near thermal equilibrium and therefore produces sample temperatures which can be measured with a degree of confidence. It has demonstrated a capability of annealing capped samples of ion implanted indium phosphide for times in the order of 20 seconds at temperatures of 700.degree. C. with a consequent marked reduction in the motion of implanted and residual impurities. Because a sample and the carrier are heated by a movable furnace, which are exposed to ambient air, some impurities can be introduced that could degrade some materials.
Thus, a continuing need exists in the state of the art for an apparatus and method of annealing a semiconductor sample that assures an exposure of a sample at a precise temperature for a precisely measured period in a nonreactive gas medium.